1. Field of the Invention
This invention, in the production of a (meth)acrylic acid comprising a step of absorbing the (meth)acrylic acid from the reaction gas obtained by the reaction of catalytic gas phase oxidation, relates to a method for preventing an effluent gas pipe from blocking, characterized by subjecting a discharged gas generated during the step of absorption mentioned above to temperature elevation and pressure application during the process of discarding the waste gas or circulating the discharged gas to a reactor used for the catalytic gas phase reaction, a method for preventing an effluent gas pipe from blocking, characterized by separating and removing mist before and/or after the temperature elevation, and a method for the production of a (meth)acrylic acid, characterized by subjecting a discharged gas to temperature elevation and pressure application or putting the discharged gas to cyclic use during a step of reaction after the removal of mist.
2. Description of the Related Art
Such polymerizable substances as acrylic acid and methacrylic acid are raw materials for commercial production and are chemical substances which are produced in large amounts in a plant of a large scale. Generally, compounds often pass through various steps of absorption and purification for the purpose of obtaining products of high purity and generate waste gases at such steps.
When propylene, propane, isobutylene, and methacrolein are subjected to catalytic gas phase oxidation with a molecular oxygen-containing gas in the presence of an oxidizing catalyst, for example, the oxidation forms in addition to the (meth)acrylic acid, i.e. the target product, such substances as aldehyde which have lower boiling points than the target product, such carboxylic acids as terephthalic acid and benzoic acid which have higher boiling points than the target product, and tarry substances as by-products. For this reason, the mixed gas consequently obtained is led to a (meth)acrylic acid absorbing column and allowed to contact a (meth)acrylic acid absorbent with a view to attaining absorption and consequently obtaining a solution containing the (meth)acrylic acid and other by-products. This solution is distilled to separate the (meth)acrylic acid as a refined product. Particularly, in the reaction of catalytic gas phase oxidation of propylene, etc. mentioned above, since the (meth)acrylic acid is produced by supplying to a reactor the raw material gas obtained by combining the propylene, etc. with oxygen gas and an inert gas and adjusting them to a prescribed composition, the gas components, inert gas and oxygen gas are continuously discharged through the reactor and the top of the (meth)acrylic acid absorbing column.
The gas thus discharged can be disposed of as a waste gas through a step of combustion. The official gazette of JP-A-09-157,213, for example, discloses a method for utilizing, though not quite satisfactorily, the gas discharged through the acrylic acid absorbing column by means of recycling the gas to a step for reaction of oxidation.
The operation of leading the (meth)acrylic acid-containing gas obtained by catalytic gas phase oxidation of propylene to the absorbing column and absorbing the gas in a solution as described above is in vogue in the process for the production of (meth)acrylic acid. Then, the practice of recycling the residual gas which is obtained through the top of the absorbing column after the absorption of the solution to a reactor for catalytic gas phase oxidation with a view to being reused as a diluting gas therein or disposing the residual gas as a waste gas is also in vogue.
The discharged gas of this origin, however, entrains such subliming substances as terephthalic acid by-produced by the reaction of catalytic gas phase oxidation, such low-boiling substances as formaldehyde, such inert gases as carbon dioxide and nitrogen gas supplied to the reactor together with the raw material gas, and molecular oxygen and water. When the gas discharged through the (meth)acrylic acid absorbing column is circulated through the effluent gas pipe to the step for reaction of catalytic oxidation, therefore, the low boiling formaldehyde entrained by the waste gas is polycondensed possibly to the extent of blocking the effluent gas pipe.
Further, since the water introduced into the step of reaction and the water used during the step for absorbing (meth)acrylic acid are effluxed as the discharged gas during the step for this absorption, this discharged gas entails such hindrances as adhering to the effluent gas pipe and corroding the compressor and, particularly when suffered to adhere to the air blower, inducing degradation of the function of air blowing. Thus, the relevant devices need periodic internal inspection, removal of polycondensate, and replacement of corroding units possibly to the extent of obstructing continuous operation.
Meanwhile in the reaction of catalytic gas phase oxidation, since the raw material gas is capable of explosion and combustion, it is preferred to be supplied to the reaction in a composition which falls outside the explosion limits. This mixed gas, therefore, contains an inert gas in a high concentration. When the gas component remaining after the reaction for catalytic gas phase oxidation is released as formed into the ambient air and when the inert gas happens to be carbon dioxide, therefore, this release is naturally improper from the viewpoint of preventing global warming.
The present inventor, as a result of his diligent study of the gas component discharged from the (meth)acrylic acid absorbing column, has found that the effluent gas pipe can be prevented from blocking by subjecting the discharged gas to temperature elevation and pressure application. This invention has been perfected based on this knowledge. To be specific, the object of the invention mentioned above is accomplished by the following items (1)-(3).
(1) In a process for the production of (meth)acrylic acid comprising at least a step for the reaction of (meth)acrylic acid by catalytic gas phase oxidation and a step for the absorption thereof, a method for preventing an effluent gas pipe from being blocked, characterized by causing part or the whole of the gas discharged from the step of absorption, while being circulated via an effluent gas pipe to the step of reaction or being discarded, to be maintained at the same temperature as the temperature of the gas discharged from the step of absorption or to be elevated to a higher temperature.
(2) In a process for the production of (meth)acrylic acid comprising at least a step for the reaction of (meth)acrylic acid by catalytic gas phase oxidation and a step for the absorption thereof, a method for preventing an effluent gas pipe from being blocked, characterized by causing part or the whole of the gas discharged from the step of absorption, while being circulated via said effluent gas pipe to the step of reaction or being discarded, to be stripped of mist.
(3) A method for the production of (meth)acrylic acid comprising at least a step for the reaction of (meth)acrylic acid by catalytic gas phase oxidation and a step for the absorption the formed acid, characterized by causing part or the whole of the gas discharged from the step of absorption at an outlet temperature of the step in the range of 30-70xc2x0 C., while being circulated via an effluent gas pipe to the step of reaction or being discarded, to be maintained at the same temperature as the temperature of the gas discharged from the step of absorption or to be heated to a higher temperature and to be pressurized.
By setting the temperature of the gas discharged from the step for absorbing (meth)acrylic acid at a level in a prescribed range according to this invention, it is made possible to adjust the concentrations of water, low boiling substance, and subliming substance contained in the discharged gas in a preferred range and consequently prevent these defiling substances from blocking the effluent gas pipe. Particularly by elevating the temperature of the interior of the effluent gas pipe, the low boiling substance and the subliming substance are enabled to remain in the gaseous state. By moreover securing the linear velocity of the gas at a prescribed level, the effluent gas pipe possibly containing a bent part is prevented from adhesion of water, low boiling substance, and subliming substance and consequently allowed to attain effective prevention of blocking.
Further by setting the temperature of the gas discharged from the step for absorbing (meth)acrylic acid at a level in a prescribed range according to this invention, it is made possible to adjust the contents of the low boiling substance and the subliming substance in the (meth)acrylic acid-containing solution occurring in the absorption column in a preferred range, and consequently prevent the (meth)acrylic acid-containing solution from encountering the blockage caused by such defiling substances in the subsequent step for refining. What is more, since it is made possible to adjust the discharged gas, the low boiling substance and the subliming substance in a preferred range, it can be reused effectively at the step of reaction.
A finned-tube type exchanger or a plate-type exchanger is used particularly effectively for elevating the temperature of the gas in the effluent gas pipe which connects the absorption column to the step of reaction because it permits the temperature to be elevated to the target level in a particularly short period of time.
By stripping the discharged gas from the absorption column of the mist according to this invention, the otherwise possible deposition of scale on the pipes and the devices which are disposed subsequently to the absorption column can be effectively prevented. Consequently, the apparatus for the production of (meth)acrylic acid can be stably operated for a long time.